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Magnetic tapes

10 Magnetic tape is a long
plastic strip coated with a magnetic substance
such as iron oxide, which works on the same principles as those that operate when you use a cassette tape in your
walkman. Information can be written to
the tape by the read/write head of a tape drive, and then stored as magnetised spots on the tape's surface.
These spots can then be read by the same
read/write head of the tape drive, converted
to information that the computer can understand and sent to the CPU. Three types of tape are used:

· reel-to-reel tape

· cassette tape

· cartridge tape

Reel-to-reel
tape

11 A typical reel-to-reel
tape is about 800m long, 1 cm wide and contained on a reel about 30 cm in diameter. These tapes can hold large amounts of information and are used with the larger
computer systems.

Cassette tape

12 Cassette tape is the familiar tape that
we have all used and is about j 8 mm wide, comes in various lengths and is
contained inside a plastic case. The early microcomputers used cassette tapes
for backing store.

Cartridge tape

13 A cartridge looks like a large cassette but
with wider and longer tape. Cartridge tapes are used both by large computer
systems and microcomputers. A typical use with a microcomputer would be to
create a copy of valuable information as a safety measure in case it were lost
from its normal media storage. This procedure is known as backing up and should
be done regularly in all computer systems.

14 Magnetic tapes suffer
from one disadvantage: information is stored on them sequentially-one piece of
information following another, so if a particular piece of information that is
required is stored at the far end of the tape, the tape drive has to wind all
the way through the tape before the required information can be read. This
slows down the process of information retrieval, but this disadvantage has been
overcome by the introduction of magnetic disks.

Tapes were used to store
information long before the advent of computers, for example with ticker-tape
and dictaphone machines.

15 A magnetic disk is a circular
sheet of plastic coated with the same magnetic
material as the magnetic tape. Its advantages over other forms of secondary
storage are:

·
access of
information;

· speed of information transfer;

· amount of information stored.

Access of information

16 Information is
contained on the disk surface in a series of concentric, circular tracks. The read/write head can move
backwards and forwards along the radius of the disk, thereby making it possible
to access information directly.

Speed of
information transfer

17 Because the
disk can spin at a much faster rate than a tape can move past the tape reader's
read/write head, information stored on a disk can be accessed at a much faster
rate.

Amount of information stored

18 The speed at which the disk rotates also
permits information to be more densely packed onto the disk surface. In this
way a disk can hold far more information than a tape.

Find out how much information
the disks that you use with your computer can store.

There are three different types of
disk.

· Floppy Diskettes

· Hard Disks

· Disk Packs

Floppy
diskettes

19 A floppy diskette is either 8,
5.25 or 3.5 inches in diameter and is contained inside a stiff plastk jacket.
It may be single sided-information only being stored on one side-or double
sided in which case both sides can be used. The magnetic material on the disk
surface may be
single, double or х4 density.
The denset the material, the more information can be stored on the disk. A
floppy diskette is used with a disk drive into which the diskette is inserted.
The disk drive may have one or two read/write heads. A two headed disk drive is
capable of
reading from and writing to both sides of the diskette without having toextract
the diskette and turn it over. Floppy diskettes are standard means of storing
information for use with a microcomputer.

Hard disks

20 A hard disk is made from aluminium instead of plastic
and because of its rigidity can store more closely packed information than a
floppy diskette. It can also be rotated at a much faster rate than a floppy
diskette, thereby permitting an even faster rate of information access. Hard
disks come in a variety of forms.